The present invention relates to oximeter sensors, in particular to pulse oximeter sensors with a colored contact surface.
In a conventional oximetry sensor, light is typically emitted by red and infrared emitters and detected by a photodetector. The light scatters through the tissue between the emitters and the detector. There are two types of paths that the photons can generally take. One type of photon path resides mostly inside the blood-perfused tissue without seeing the effects of the skin. The other type of photon path experiences a significant effect of the skin surface which contains skin pigment (melanin). It has been shown that melanin absorbs red light to a greater degree than near infrared light. Accordingly, a pulse oximeter sensor calibrated for its red and infrared emitters for a particular amount of skin pigmentation will produce an erroneous result when the sensor is applied to a patient having a different amount of skin pigmentation. The skin pigmentation will absorb more red light than infrared light, causing a greater variance in the calibration from the correct one.
For example, a pulse oximeter sensor may be calibrated for light pigmentation skin, with a sensor body surface that is white. Such a sensor will lose accuracy when it is applied to a patient having a darker skin pigmentation. One approach to addressing this problem is to use a sensor body having a dark (gray or black) colored surface. This reduces the amount of secondary scattered contribution associated with the skin pigmentation making the calibration more consistent and less dependent on the degree of skin pigmentation. However, this also reduces the strength of the pulsatile (AC) component of the signal. Accordingly, a compromise must be made between accuracy and pulsatile signal strength. The latter is desired in the case of a low-perfusion application.